Multi-layer articles can be used in a variety of applications. For example, superconductors, including oxide superconductors, can be formed of multi-layer articles. Typically, such superconductors include a layer of superconductor material and a layer, commonly referred to as a substrate, which can enhance the mechanical strength of the multi-layer article.
Generally, in addition to enhancing the strength of the multi-layer superconductor, the substrate should exhibit certain other properties. For example, the substrate should have a low Curie temperature so that the substrate is not ferromagnetic at the superconductor""s application temperature. Furthermore, chemical species within the substrate should not be able to diffuse into the layer of superconductor material, and the coefficient of thermal expansion of the substrate should be about the same as the superconductor material. Moreover, if the substrate is used for an oxide superconductor, the substrate material should be relatively resistant to oxidation.
For some materials, such as yttrium-barium-copper-oxide (YBCO), the ability of the material to provide high transport current in its superconducting state depends upon the crystallographic orientation of the material. For example, such a material can exhibit a relatively high critical current density (Jc) when the surface of the material is biaxially textured.
As used herein, xe2x80x9cbiaxially texturedxe2x80x9d refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface. One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Examples of cube textured surfaces include the (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface.
For certain multi-layer superconductors, the layer of superconductor material is an epitaxial layer. As used herein, xe2x80x9cepitaxial layerxe2x80x9d refers to a layer of material whose crystallographic orientation is directly related to the crystallographic orientation of the surface of a layer of material onto which the epitaxial layer is deposited. For example, for a multi-layer superconductor having an epitaxial layer of superconductor material deposited onto a substrate, the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the substrate. Thus, in addition to the above-discussed properties of a substrate, it can be also desirable for a substrate to have a biaxially textured surface or a cube textured surface.
Some substrates do not readily exhibit all the above-noted features, so one or more intermediate layers, commonly referred to as buffer layers, can be disposed between the substrate and the superconductor layer. The buffer layer(s) can be more resistant to oxidation than the substrate, and reduce the diffusion of chemical species between the substrate and the superconductor layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion that is well matched with the superconductor material.
One type of buffer layer, commonly referred to as a seed layer, is the buffer layer that is formed on the surface of the substrate. Another type of buffer layer, commonly referred to as a cap layer, is the buffer layer on which the superconductor material is formed. A further type of layer, commonly referred to as a barrier layer, is typically a relatively thick layer and acts to substantially reduce diffusion of constituents from the substrate into the superconductor material. In some embodiments, a multi-layer superconductor article contains a seed layer, a cap layer and a barrier layer. In certain embodiments, a multi-layer superconductor article contains a buffer layer that serves as the seed layer, the barrier layer and the cap layer.
Typically, a buffer layer is an epitaxial layer, so its crystallographic orientation is directly related to the crystallographic orientation of the surface onto which the buffer layer is deposited. For example, in a multi-layer superconductor having a substrate, an epitaxial buffer layer and an epitaxial layer of superconductor material, the crystallographic orientation of the surface of the buffer layer is directly related to the crystallographic orientation of the surface of the substrate, and the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the surface of the buffer layer. Therefore, the superconducting properties exhibited by a multi-layer superconductor having a buffer layer can depend upon the crystallographic orientation of the buffer layer surface.
The termination plane of a buffer layer can include one or more regions having an undesirable orientation.
The invention relates to multi-layer articles (e.g., multi-layer superconductors) and methods of making such multi-layer articles.
Typically, the articles have one or more layers (e.g., a substrate, a buffer layer, a superconductor layer) having a termination plane with a relatively high degree of orientation in a desirable direction (e.g., (001)). xe2x80x9cTermination planexe2x80x9d of a layer as used herein refers to the plane of the layer that forms an interface with another layer that is formed thereon. These planes can be prepared, for example, by one or more of the processes disclosed herein.
In general, the methods involve conditioning (e.g., by chemical conditioning and/or thermal conditioning) the surface of an underlying layer (e.g., a buffer layer or a superconductor material layer), and then depositing a layer of material (e.g., a superconductor material, a precursor of a superconductor material, a cap material or a buffer layer material), onto the conditioned surface of the underlying layer. The conditioned surface can be relatively atomically flat and/or have a termination plane with a relatively high amount of a desired orientation. A multi-layer superconductor prepared by the methods of the invention can exhibit a relatively high critical current density (Jc). A multi-layer superconductor prepared by the methods of the invention can be formed into an object having a relatively large surface area, such as a wafer or a tape.
xe2x80x9cChemical conditioningxe2x80x9d as used herein refers to a process which uses one or more chemical species (e.g., gas phase chemical species and/or solution phase chemical species) to affect changes in the surface of a material layer, such as a buffer layer or a superconductor material layer.
xe2x80x9cThermal conditioningxe2x80x9d as used herein refers to a process which uses elevated temperature, with or without chemical conditioning, to affect changes in the surface of a material layer, such as a buffer layer or a superconductor material layer. Preferably, thermal conditioning occurs in a controlled environment (e.g., temperature and pressure).
In one aspect, the invention features an article that includes a substrate, a layer of a buffer material supported by the termination plane of the substrate, and a layer of a superconductor material disposed on the termination plane of the layer of the buffer material. At least about 25% (e.g., at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%) of the termination plane of the layer of the buffer material is the (001) plane.
In another aspect, the invention features an article that includes a substrate, a layer of a buffer material supported by the termination plane of the substrate, and a layer of a superconductor material disposed on the termination plane of the layer of the buffer material. At most about 75% (e.g., at most about 50%, at most about 25%, at most about 10%, at most about 1%) of the termination plane of the layer of the buffer material comprising the (111) plane.
In a further aspect, the invention features an article that includes a substrate, a layer of a buffer material supported by the termination plane of the substrate, and a layer of a superconductor material disposed on the termination plane of the layer of the buffer material. A ratio of the amount of the termination plane of the layer of the buffer material that is the (111) plane to the amount of the termination plane of the buffer material that is the (001) plane is less than about one (e.g. less than about 0.75, less than about 0.5, less than about 0.4, less than about 0.3, less than about 0.2, less than about 0.1).
Embodiments can include one or more of the following features.
The termination plane of the layer of the buffer material can have at least one (001) plateau with an area of at least about 500 square nanometers.
The layer of the buffer material can have a thickness of at least about five nanometers.
The termination plane of the layer of the buffer material can be parallel to the termination plane of the substrate to within about 5xc2x0.
The buffer material can be, for example, a cap material. The cap material can have, for example, a fluorite or perovskite crystallographic structure. The cap material can be, for example, CeO2, Y2O3, TbOx, GaOx, YSZ, LaAlO3, Gd2O3, LaNiO3, LaCuO3, SrTiO3, NdGaO3, NdAlO3, MgO, CaF2, AlN, NbN, TiN, VN, ZrN, NiO, Ag, Al2O3 SmBa2Cu3Ox, MgF2, LaMnO3, La0.66Ca0.33MnO3, La0.66Sr0.33MnO3 La0.66Ba0.33MnO3, La2Zr2O7, Gd2Zr2O7, Ba2Zr2O7 or doped compounds thereof. The cap material can be, for example, doped with at least one rare earth metal.
The bulk portion of the buffer material can have a texture selected from the group consisting of biaxial texture and cube texture.
The article can further include at least one additional buffer layer between the substrate and the layer of the superconductor material.
The superconductor material can be a rare earth oxide superconductor material. The rare earth oxide superconductor material comprises YBCO.
In one aspect, the invention features a method of making a multi-layer article which includes chemically conditioning the surface of a buffer layer or a superconductor layer to form a conditioned surface, and disposing a layer of a second material on the conditioned surface.
In another aspect, the invention features a method of making a multi-layer article, which includes heating the surface of a buffer layer or a superconductor layer to a temperature at least about 5xc2x0 C. above the deposition temperature or the crystallization temperature of the layer. The layer is heated at an oxygen gas pressure of less than about 700 Torr to form a conditioned surface. The method also includes disposing a second material layer on the conditioned surface.
xe2x80x9cDeposition temperaturexe2x80x9d as used herein refers to the temperature at which the layer being conditioned was deposited.
xe2x80x9cCrystallization temperaturexe2x80x9d as used herein refers to the temperature at which a layer of initially non-crystalline or amorphous material takes on a crystalline form.
In a further aspect, the invention features a method of making a multi-layer article which includes heating the surface of a buffer layer or a superconductor layer to a temperature at least about 5xc2x0 C. above the crystallization or deposition temperature of the layer to form a conditioned surface. The buffer layer or superconductor layer is disposed on the surface of a polycrystalline material. The method further includes disposing a second material layer on the conditioned surface.
Examples of polycrystalline materials include materials formed by deformation texturing and materials formed by vacuum deposition (e.g., ion beam assisted deposition).
In certain embodiments, two or more of the foregoing methods can be combined.
The conditioned surface can be biaxially textured.
The second material can be a buffer material, a cap material, a superconductor material or a precursor of a superconductor material.
The multi-layer articles can include additional layers of material, such as, for example, superconductor material layers and/or buffer material layers. The surface of one or more of the additional layers can be conditioned as described herein and can exhibit the properties of a conditioned surface as described herein.
The multi-layer articles can have one or more layers of superconductor material with a critical current density of at least about 1xc3x97106 Amperes per square centimeter.
One potential advantage of the invention is that it can provide multi-layer superconductors having one or more layers of superconductor material with a relatively high critical current density, and methods of making such superconductors.
Another potential advantage of the invention is that it can provide a multi-layer article (e.g., a multi-layer superconductor), as well as methods of making such a multi-layer article, having a buffer layer and/or a superconductor layer with a high quality surface. For example, the surface can be relatively atomically flat (e.g., as measured by atomic force microscopy) and/or have an external plane with a relatively high amount of a desired orientation.
An additional potential advantage of the invention is that it can provide methods of making multi-layer articles (e.g., multi-layer superconductors) which can be performed using a wide range of conditions (e.g., temperature, gas environment and gas pressure).
A further potential advantage of the invention is that it can provide methods of making a multi-layer article (e.g., a multi-layer superconductor) without using a high oxygen pressure during preparation of one or more underlying material layers, such as a buffer layer and/or a layer of superconductor material. This can allow relatively oxygen sensitive materials, such as tungsten, copper, chromium and/or nickel, to be used as the underlying layer (e.g., the substrate).
Still a further potential advantage of the invention is that it can provide methods of making a multi-layer article (e.g., a multi-layer superconductor) while using chemical conditioning to condition the surface of one or more underlying material layers, such as during the preparation of a buffer layer and/or a layer of superconductor material.
Another potential advantage of the invention is that it can provide methods of making a multi-layer superconductor in which the superconductor material, or a precursor thereof, can be disposed on a conditioned surface of an underlying layer (e.g., a superconductor material layer or a buffer layer) without first annealing the conditioned surface.
In embodiments in which multiple underlying layers (e.g. multiple buffer layers and/or multiple superconductor material layers) are used in the multi-layer article, the foregoing methods and/or advantages can apply to one or more of the multiple underlying layers.
In some embodiments, the superconductor material is preferably formed of YBCO (e.g., YBa2Cu3O7xe2x88x92x).
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features, aspects and advantages of the invention will be apparent from the figures, the description of the preferred embodiments thereof and the claims.